This invention relates to a liquid jetting apparatus for jetting liquid of ink, glue, manicure, etc., through nozzle orifices and in particular to an apparatus intended for preventing liquid in nozzle orifices from being increased in viscosity.
Related arts will be discussed by taking an ink jet recording apparatus as one example of a liquid jetting apparatus. To record an image or a character on recording paper with an ink jet recording apparatus such as a printer or plotter, a recording head is moved in a main scanning direction and recording paper is moved in a subscanning direction and ink drops are jetted through nozzle orifices in association with their move. The ink drops are jetted, for example, by causing pressure variation to occur in liquid in pressure chambers communicating with the nozzle orifices.
In the nozzle orifices of the recording head, a meniscus, namely, a free surface of ink exposed on the nozzle orifices is exposed to air, thus an ink solvent (for example, water) evaporates gradually. If the ink viscosity in the nozzle orifices rises as the ink solvent evaporates, a problem of flying an ink drop in a direction deviated from the normal direction, etc., occurs. Thus, in the ink jet recording apparatus, countermeasures to prevent ink drops in the nozzle orifices from being increased in viscosity are taken. One of the countermeasures against an increase in viscosity of the ink drops is agitation of slight vibration of meniscuses.
In agitation, a vibration pulse signal is applied to a pressure generating element for causing pressure variation to occur in liquid in a pressure chamber and a meniscus is slightly moved (vibrated) in a jetting direction and an opposite direction thereof. As the meniscus is finely vibrated, ink in the nozzle orifice is mixed with any other ink in the pressure chamber for preventing ink from being increased in viscosity. Such agitation of ink is executed in association with the record operation. For example, it is executed during acceleration period just after main scanning of a carriage on which the recording head is mounted is started or during the one-line recording period. In agitation in the recording period (in-print vibration), a vibration pulse signal contained in a drive signal is selected and is supplied to the recording head.
By the way, for this kind of ink jet recording apparatus, improvements in the image quality and the recording speed are demanded. To attain high image quality, gradation representation with small dots is effective, and to speed up recording, record with large dots is effective. That is, to provide compatibility between high quality of a record image and speeding up of recording, it is useful to jet an ink drop capable of forming a small dot and an ink drop capable of forming a large dot through the same nozzle orifice.
Then, the following is considered: More than one ejection pulse signal capable of jetting a small amount of ink drop is contained in one recording period to make up a drive signal sequence and the ejection pulse signals are selectively applied to the recording head, whereby the volume of each ink drop jetted is changed. For example, three ejection pulse signals each for jetting a small ink drop of 13.3 pL (picoliters) are contained in one recording period (7.2 kHz) to make up a drive signal. The small ink drops are selectively jetted, whereby gradation representation is provided. On the other hand, to record at high speed, the three small ink drops are all jetted for recording a large dot on recording paper.
By the way, this kind of ink jet recording apparatus involves demand for furthermore speeding up record. To meet this demand, one recording period needs to be shortened as much as possible. However, it is difficult to shorten one recording period in a case where a plurality of ejection pulse signals and vibration pulse signals are simply connected. To use ink with relatively fast viscosity increase speed, such as pigment-family ink in contrast to dye-family ink, to jet minute ink drops, vibration of agitating ink in the vicinity of each nozzle orifice becomes indispensable for preventing an ink jet failure caused by an increase in ink viscosity.
It is therefore an object of the invention to provide a liquid jetting apparatus capable of shortening the repetition cycle of a drive signal while preventing liquid in the vicinity of a nozzle orifice from being increased in viscosity.
In order to achieve the above object, according to the present invention, a vibration pulse signal is separated into a pressure reducing element for reducing pressure of liquid in the pressure chamber to such an extent that a liquid drop is not ejected and a pressure increasing element for increasing pressure of liquid in the pressure chamber to such an extent that a liquid drop is not ejected. A drive signal sequence comprises at least one ejection element placed between the pressure reducing element and the pressure increasing element. The pressure reducing element and the pressure increasing element are selectively applied to the pressure generating element, thereby finely vibrating a meniscus. Thus, the time required for the pressure reducing element and the pressure increasing element mainly depend on the time of the gradient portion thereof.
Thus, if a plurality of ejection pulse signals and vibration pulse signals are mixed to make up a drive signal sequence, one unit printing period can be placed within a short time. Therefore, the repetition cycle of a drive signal can be shortened while liquid in the vicinity of a nozzle orifice is prevented from being increased in viscosity.
A sufficient time can be provided from application termination of the pressure reducing element to application start of the pressure increasing element. Thus, vibration caused by the waveform of one of the pressure reducing element and the pressure increasing element is settled to some extent before vibration caused by the waveform of the other can be started. Therefore, vibration of a meniscus can be carried out reliably without jetting any liquid drop.
The drive signal generated by the drive signal generator is a signal comprising at least the waveform of one of the pressure reducing element and the pressure increasing element placed between adjacent ejection pulse signals, so that the time between the ejection pulse signals which must be set to a relatively long time can be used effectively and if the jet drive and vibration pulse signals are mixed in the drive signal, one unit printing period can be placed within a short time.
The drive signal generated by the drive signal generator is a signal wherein at least either different potential levels between the pressure reducing element and the ejection pulse signal or different potential levels between the pressure increasing element and the ejection pulse signal are jointed by a connection element not applied to the pressure generating element, so that the time required for the connection element can be shortened as much as possible and the jet drive and vibration pulse signals can be mixed efficiently within one short unit printing period.
The invention can be embodied in various forms of a printing method, a printer, a computer program for providing the function of the printing method or the printer, a data signal containing the computer program which is provided in a carrier wave, and the like.